Multi-dimensional ion mobility spectrometry methods and apparatus

ABSTRACT

Various embodiments of a multi-dimensional ion mobility analyzer are disclosed that have more than one drift chamber and can acquire multi-dimensional ion mobility profiles of substances. The drift chambers of this device can, for example, be operated under independent operational conditions to separate charged particles based on their distinguishable chemical/physical properties. The first dimension drift chamber of this device can be used either as a storage device, a reaction chamber, and/or a drift chamber according to the operational mode of the analyzer. Also presented are various methods of operating an ion mobility spectrometer including, but not limited to, a continuous first dimension ionization methods that can enable ionization of all chemical components in the sample regardless their charge affinity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 12/471,101, filed May 22,2009, now granted, which is a continuation of U.S. patent applicationSer. No. 11/618,430, filed Dec. 29, 2006, the latter now issued as U.S.Pat. No. 7,576,321, which claims priority from Provisional Application60/766,266, filed Jan. 2, 2006. The contents of all of these patentapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Since it was invented in the early 1970's, ion mobility spectrometry(IMS) has been developed into a powerful analytical tool used in avariety of applications. There are three major forms of this instrumentincluding independent chemical detection systems, chromatographicdetectors, or hyphenated IMS mass spectrometry (MS) systems. As anindependent detection system, IMS qualitatively and quantitativelydetects substances in different forms relying on its capability toionize the target substance, to separate the target substance frombackground based on interactions with a drift gas (i.e. a carrier gas),and to detect the substance in its ionized form. As a chromatographicdetector, IMS acquires multiple ion mobility spectra ofchromatographically separated substances. In combined IMS-MS systems,IMS is used as a separation method to isolate target substances beforemass analysis. However, the resolution of IMS is generally consider low,often regulating such devices to qualitative use or use in environmentswith low levels of interferants with respect to the substances ofinterest.

The basic common components of an IMS system consist of an ionizationsource, a drift tube that includes a reaction region, an ion shuttergrid, a drift region, and an ion detector. In gas phase analysis thesample to be analyzed is introduced into the reaction region by an inertcarrier gas, ionization of the sample is often completed by passing thesample through a reaction region and/or a radioactive 63Ni source. Theions that are formed are directed toward the drift region by an electricfield applied to drift rings that establish the drift region, and anarrow pulse of ions is then injected into, and/or allowed to enter, thedrift region via an ion shutter grid. Once in the drift region, ions ofthe sample are separated based upon their ion mobilities and therearrival time at a detector is an indication of ion mobility which can berelated to ion mass. However, it is to be understood that ion mobilityis not only related to ion mass, but rather is fundamentally related tothe ion-drift gas interaction potential which is not solely dependent onion mass.

One of the major applications of IMS is to detect trace amounts ofcontraband chemicals. The trace detection system has been widely used incurrent security systems for explosive and chemical agent detections.Typically, the process starts when a security officer wipes a swab overa sampling surface, and then inserts the swab into a thermal desorberwhere traces of organic compounds are evaporated and introduced to theIMS. In most of these applications fast and accurate identification ofcontraband chemicals is essential to the security inspection mission.Portable yet high performance detection systems continue to be soughtafter and are highly desirable.

SUMMARY OF THE INVENTION

The present invention relates to various aspects of Multi-DimensionalIon Mobility Spectrometry (MDIMS) methods and apparatus. In variousembodiments, the MDIMS of the present inventions differentiatethemselves from conventional ion mobility spectrometry (IMS) byinnovatively integrating multiple ion mobility based separation steps inone device. In various embodiments, the present invention provideshigher resolution and higher sensitivity than conventional IMS devicesand operational approaches.

Various embodiments of the present invention provide an integratedmultiple dimensional time-of-flight ion mobility spectrometric systemthat ionizes, separates, and detects chemical species based on their ionmobilities. These systems generally include: (a) at least one ionizationsource, (b) at least two drift regions, and (c) at least one iondetection device. In various embodiments, these systems separate ions inone drift dimension under one set of drift conditions; and subsequently,the separated ions are introduced into a higher dimension for furtherseparation under the same or a different set of drift conditions. Invarious embodiments, the separation process can be repeated for one ormore additional drift dimensions. Also, in various embodiments, thefirst drift dimension is used as one or more of an ionization source,reaction region or desolvation region, and drift region for the system.For example, in various embodiments, the electric field in the firstdrift dimension (first drift tube) can be used as a desolvation regionfor charged droplets.

The devices and methods of the present inventions make use of “drifttubes.” The term “drift tube” is used herein in accordance with theaccepted meaning of that term in the field of ion mobility spectrometry.A drift tube is a structure containing a neutral gas through which ionsare moved under the influence of an electrical field. It is to beunderstood that a “drift tube” does not need to be in the form of a tubeor cylinder. As understood in the art, a “drift tube” is not limited tothe circular or elliptical cross-sections found in a cylinder, but canhave any cross-sectional shape including, but not limited to, square,rectangular, circular, elliptical, semi-circular, triangular, etc.

Neutral gas is often referred to as a carrier gas, drift gas, buffergas, etc. and these terms are considered interchangeable herein. The gasis at a pressure such that the mean free path of the ion, or ions, ofinterest is less than the dimensions of the drift tube. That is the gaspressure is chosen for viscous flow. Under conditions of viscous flow ofa gas in a channel, conditions are such that the mean free path is verysmall compared with the transverse dimensions of the channel. At thesepressures the flow characteristics are determined mainly by collisionsbetween the gas molecules, i.e. the viscosity of the gas. The flow maybe laminar or turbulent. It is preferred that the pressure in the drifttube is high enough that ions will travel a negligible distance,relative to the longitudinal length of the drift tube, before asteady-state ion mobility is achieved.

The axis of the drift tube along which ions move under the influence ofthe electrical drift field is referred to herein as a drift axis. Thedrift axis is often, but not necessarily, a longitudinal axis of thedrift tube.

In various aspects of the present inventions, methods for operating anion mobility spectrometer are described. In one aspect, methods ofoperation referred to for the sake of conciseness, and not by way oflimitation, as Continuous First Dimension Ionization (CFDI) mode aredescribed. As understood in the art, charge will preferentially betransferred from an ionized chemical species to another chemical speciesof higher affinity. Such charge transfer process can seriously disturband even prevent the ionization of a chemical species of interest, andhence prevent successful analysis of that species by IMS as well asother forms of mass spectrometry. The CFDI methods of the presentinventions facilitate the formation of ions of low charge affinity ionsand thus, in various embodiments, can increase the sensitivity of IMS.The CFDI method also increases the dynamic response range of the IMS andprovides better quantitative information when the spectrometer is usedto analyze sample mixture.

In various embodiments, a CFDI method comprises pulsing a gas phasesample into a first drift tube and conveying the sample pulse by gasflow in a first direction along at least a portion of the first drifttube. As a result, the first direction is substantially parallel to thedirection of carrier gas flow in the first drift tube. Pulses ofcounter-moving reactant ions are used to ionize chemical species in thesample pulse. The sample pulse can be introduced to the spectrometereither in the reaction region or in the drift region. In some modes ofoperation, the carrier gas and sample pulse can have a speed approachingto zero, but have the reactant ions moving toward sample pulse. Forexample, a first group of reactant ions are pulsed into the first drifttube and conveyed by the first drift tube electrical field in a seconddirection that is towards the sample pulse. The first group of reactantions is preferably pulsed into the first drift tube at a predeterminedtime. The predetermined time can be chosen to select, at least roughly,the position in the first drift tube where the sample pulse and firstgroup of reactant ions interact. As the first group of reactant ionsinteracts with the sample pulse, one or more chemical species areionized and a first ionized chemical species is produced. Typically, thechemical species in the sample pulse with the highest charge affinity ispreferentially ionized.

A second group of reactant ions is pulsed into the drift tube after thefirst group, also preferably at a second predetermined time, andconveyed by the electrical field of the first drift tube towards thesample pulse. The second predetermined time, which is necessarily laterthan that chosen for the first reactant ion group, can be chosen toselect, at least roughly, the position in the first drift tube where thesample pulse and first group of reactant ions interact. In variousembodiments, the second predetermined time is chosen to allow time forat least a portion of the first ionized chemical species to be extractedinto a second drift tube.

As the second group of reactant ions interacts with the sample pulse,one or more chemical species are ionized and a second ionized chemicalspecies is produced. Typically, the second chemical species ionized isthe species in the sample pulse with the second highest charge affinity.The process can be repeated, e.g., providing a third group of reactantions to produced a third ionized chemical species, a further group ofreactant ions to produced a fourth ionized chemical species, etc. as theoperator desires, to ionize a chemical species of interest.

In various embodiments, at least a portion of the first ionized chemicalspecies in the first drift tube is extracted into a second drift tube bygenerating an electrical field over at least a portion of the firstdrift tube that moves the ions into the second drift tube. Thiselectrical field is often referred to herein as a “kick-out” pulse or“kick-out” field as it removes the ions from the first drift tubes.Preferably, the kick-out field is applied to a portion of the firstdrift tube that is substantially free of reactant ions, by selecting,for example, the timing, spatial extent, or both at which field isapplied. For example, in various embodiments, the kick-out field isapplied prior to the ionization of a second chemical species by thesecond group of reactant ions.

In various embodiments, a kick-out field is applied to extract a nthionized chemical species prior to formation of the next, (n+1)th,ionized chemical species. In various embodiments, a kick-out field isapplied to extract two or more ionized chemical species at substantiallythe same time. In various embodiments, a combination of selectiveionized chemical species extraction and multiple ionized chemicalspecies extraction is performed.

Accordingly, in various embodiments, a CFDI method of the presentinventions comprises: (a) pulsing a gas phase sample into a first drifttube at a first time; (b) conveying the gas phase sample pulse in afirst direction along at least a portion of the first drift tube,wherein the first direction is substantially parallel to the directionof carrier gas flow in the first drift tube; (c) providing a pluralityof pulses of reactant ions into the first drift tube at predeterminedtimes relative to the first time; (d) conveying by an electrical fieldthe pulses of reactant ions in a second direction along at least aportion of the first drift tube, wherein the second direction issubstantially anti-parallel to the direction of carrier gas flow in thefirst drift tube; (e) interacting a group of reactant ions, comprisingone or more of the plurality of pulses of reactant ions, with the gasphase sample pulse to ionize a chemical species in the gas phase samplepulse; and (f) repeating the step of interacting a group of reactantions with the gas phase sample pulse until all chemical species ofinterest are ionized, wherein chemical species of different chargeaffinity are ionized at different positions along the first direction.

In various embodiments, the methods comprise extracting at least aportion of the ionized chemical species in the first drift tube into asecond drift tube by generating at one or more predetermined extractiontimes an electrical field over at least a portion of the first drifttube, the second drift tube having a longitudinal axis which issubstantially parallel or perpendicular to a longitudinal axis of thefirst drift tube. Preferably, the one or more predetermined extractiontimes are selected such that ionized chemical species are extracted in atime interval during which the portion of the first drift tube fromwhich ionized chemical species are extracted is substantially free ofreactant ions.

In various embodiments, a CFDI method of the present inventions use asecond drift tube; preferably the second drift tube has a longitudinalaxis which is substantially perpendicular to a longitudinal axis of thefirst drift tube. In various embodiments, ionized chemical speciesextracted into the second drift tube are directed towards an iondetector and are characterized by their arrival time at the ion detectorbased at least on their mobility under the conditions of the seconddrift tube.

Preferably, the ratio of the electrical field strength to the gas numberdensity (E/N value) is substantially constant in the first drift tube,the second drift tube, or both. Preferably the electrical field strengthis substantially constant in the first drift tube, the second drifttube, or both. It is to be understood, however, that the conditions inthe first and second drift tubes can be different. For example, invarious embodiments, one or more of the carrier gas, carrier gasdensity, carrier gas flow rate, electrical field strength, andtemperature, are different in the first and second drift tubes.

In various embodiments, the CFDI methods of the present inventions havea variety of practical applications. For example, a continuous firstdimension ionization method of operation of a MDIMS could be used tofacilitate overcoming a fundamental shortcoming of conventional IMS,i.e., charge competition in the ionization source and/or reaction regionof the spectrometer, thus in various embodiments offering an ionizationopportunity for substances with very different charge affinities.Various embodiments of the CFDI methods could be used, e.g., to isolatecharged substances and prevent them from losing charges to otherco-existing substances; and thus facilitate increasing systemsensitivity to the substances of interest.

In various aspects, the present invention provides an ion mobilityspectrometer comprising three drift tubes and methods for suchapparatus. In one aspect, provided are methods of operation referred tofor the sake of conciseness, and not by way of limitation, as DualPolarity Ion Extraction (DPIE) methods.

In various embodiments, the present inventions provide amulti-dimensional ion mobility spectrometer that comprises three drifttubes, a first drift tube for performing a first dimension of IMS, ionformation (e.g., by CFDI), or both, and two additional drift tubes (asecond and third drift tube) for performing a second dimension of IMS.In various embodiments, the second and third drift tubes are configuredand operated to perform a second dimension of IMS on differentpolarities of ions; IMS under different drift tube conditions, or both.Examples of various embodiments of such MDIMS of the present inventionsare schematically illustrated, for example, in FIGS. 1, and 6-9. For thesake of conciseness, and not by way of limitation, we refer to such ionmobility spectrometer systems as Dual Second Dimension Ion MobilitySpectrometers (DSDIMS).

Accordingly, in various embodiments, the present invention provide anion mobility spectrometer that comprises: (a) a first drift tube havinga first drift axis; (b) a second drift tube having a second drift axissubstantially perpendicular to the first drift axis and an inlet influid communication with the first drift tube; (c) a third drift tubehaving a third drift axis substantially parallel to the second driftaxis and substantially perpendicular to the first drift axis, and havingan inlet in fluid communication with the first drift tube; (d) anelectrode arranged opposite the inlet of the second drift tube; and (e)an electrode arranged opposite the inlet of the third drift tube.

The second and third drift tubes can be arranged in a variety of ways.In various embodiments, the second drift tube and third drift tube arearranged substantially opposite each other across from the first drifttube. An example of such embodiments includes, but is not limited to,the embodiments schematically illustrated in FIG. 8. The opposedarrangement of the second and third drift tubes can provide structureswhere the electrode arranged opposite to the inlet of the second drifttube comprises the inlet to the third drift tube; and the electrodearranged opposite the inlet of the third drift tube comprises the inletto the second drift tube.

In various embodiments, the second drift tube and third drift tube arearranged substantially side-by-side. Examples of such embodimentsinclude, but are not limited to, the embodiments schematicallyillustrated in FIGS. 1, 6, 7, and 9. In various embodiments, theproximity of the second and third drift tubes leads to a preferredembodiments where the electrode arranged opposite to the inlet of thesecond drift tube is the same structure as the electrode arrangedopposite to the inlet of the third drift tube. For example, in variousembodiments, an electrical potential V is applied to this electrode anda lower electrical potential is applied to the inlet of the second drifttube. A higher potential is applied to the inlet of the third drift tubein order to attract ions of different polarity to the second and thirddrift tubes.

The Dual Second Dimension Ion Mobility Spectrometers (DSDIMS) of thepresent invention can include higher dimensions of IMS. In variousembodiments, a DSDIMS of the present invention also comprises (a) afourth drift tube having an inlet in fluid communication with the seconddrift tube and having a fourth drift axis substantially perpendicular tothe second drift axis; and (b) a fifth drift tube having an inlet influid communication with the third drift tube and having a fifth driftaxis substantially perpendicular to the third drift axis. An example ofsuch embodiments includes, but is not limited to, the embodimentsschematically illustrated in FIG. 1. In various embodiments, the fourthdrift axis and the fifth drift axis are both substantially perpendicularto the first drift axis.

A wide variety of ion source and ion detector configurations arecontemplated for the multi-dimensional IMS systems of the presentinventions. For example, in various embodiments, a DSDIMS of the presentinventions also comprises an ion source in fluid communication with afirst end of the first drift tube and an ion detector located at an endof the first drift tube opposite the first end. An example of suchembodiments includes, but is not limited to, the embodimentsschematically illustrated in FIG. 1.

In various embodiments, a DSDIMS of the present inventions alsocomprises a first ion source in fluid communication with a first end ofthe first drift tube; and a second ion source in fluid communicationwith an end of the first drift tube opposite to the first end. Anexample of such embodiments includes, but is not limited to, theembodiments schematically illustrated in FIG. 7.

In various embodiments, a DSDIMS of the present inventions alsocomprises an ion source positioned between the inlet to the second drifttube and the inlet to the third drift tube. The ion source is in fluidcommunication with the first drift tube; and a first ion detectorlocated at a first end of the first drift tube. In various embodiments asecond ion detector is located at an end of the first drift tubeopposite to the first end. An example of such embodiments includes, butis not limited to, the embodiments schematically illustrated in FIG. 9.

In various aspects, the present inventions provide methods for operatinga DSDIMS. In one aspect, provided are methods of operation referred tofor the sake of conciseness, and not by way of limitation, as DualPolarity Ion Extraction (DPIE). In various embodiments, a MDIMS of thepresent inventions, including DSDIMS, can be operated to include ionstorage. For example, in various embodiments, loss of ions in ionizationand drift chambers suffered by conventional IMS designs can be reducedor avoided. For example, a MDIMS operated in DPIE mode can providesubstantially simultaneous analysis of positive and negative ions, e.g.,such as would be present in peroxide- and nitro-based explosivesdetection. Such operation can, in various embodiments, provide increasedsensitivity over conventional IMS approaches. In various embodiments, aDSDIMS configuration of the present inventions could be provided in verycompact format, suitable for hand-held instrumentation. Such hand-helddevices could find widespread use in the area of homeland security.

Accordingly, in various embodiments, the present invention providesmethods for operating an ion mobility spectrometer, such as, e.g., aDSDIMS, where the ion mobility spectrometer comprises (a) a first drifttube having a first drift axis; (b) a second drift tube having a seconddrift axis substantially perpendicular to the first drift axis; (c) athird drift tube having a third drift axis substantially parallel to thesecond drift axis and substantially perpendicular to the first driftaxis; (d) a first electrode arranged opposite the inlet of the seconddrift tube; and (e) a second electrode arranged opposite the inlet ofthe third drift tube.

The methods in various embodiments, comprise the steps of: (a) providinga gas sample comprising positive ion chemical species and negative ionchemical species to the first drift tube; (b) spatially separating alongthe first drift axis one or more of the chemical species by collisionswith a first carrier gas in the first drift tube; (c) applying anelectrical potential difference between the first electrode arrangedopposite to the inlet of the third drift tube in order to move at leasta portion of the positive ion chemical species into the second drifttube and substantially simultaneously move at least a portion of thenegative ion chemical species into the third drift tube; (d) spatiallyseparating along the second drift axis one or more of the positive ionchemical species by collisions with a second carrier gas in the seconddrift tube and conveying at least a portion of the separated positiveion chemical species to an ion detector; and (e) spatially separatingalong the third drift axis one or more of the negative ion chemicalspecies by collisions with a third carrier gas in the third drift tubeand conveying at least a portion of the separated negative ion chemicalspecies to an ion detector.

The methods can be used, for example to determine the presence orabsence of one or more chemical species in a sample, such as forexample, chemical species associated with peroxide-based explosives andnitro-based explosives, in a single measurement. Such determinations canbe made, based on the arrival time at the ion detector associated withthe second drift, the third drift tube, or both.

The drift conditions in the first second and third drift tubes can besubstantially the same or different. For example, in variousembodiments, one or more of the drift tube conditions of carrier gas,carrier gas density, carrier gas flow rate, electrical field strength,and temperature, are different between one or more of the first drifttube, the second drift tube, and the third drift tube.

In various embodiments, the use of different drift conditions is oneaspect of various methods of the present inventions. For example, forthe same sample, various embodiments of the present inventions providemeans of achieving multiple ion mobility based separations underdifferent conditions in one data acquisition cycle. Conditions includingthe type of drift gas, temperature, pressure, electric field strength,flow rate, etc.. These conditions can be adjusted to change ion mobilitybased separation characteristics of individual substances. Thus, invarious embodiments, substances irresolvable in a conventional IMS canbe separated in the MDIMS; preferably in one data acquisition cycle.

In various embodiments, the present invention provides methods foroperating an ion mobility spectrometer, such as, e.g., a DSDIMS, wherethe ion mobility spectrometer comprises: (a) a first drift tube having afirst drift axis; (b) a second drift tube having a second drift axissubstantially perpendicular to the first drift axis; (c) a third drifttube having a third drift axis substantially parallel to the seconddrift axis and substantially perpendicular to the first drift axis; (d)a first electrode arranged opposite the inlet of the second drift tube;and (b) a second electrode arranged opposite the inlet of the thirddrift tube.

The ion mobility spectrometer is operated with different driftconditions for the second drift tube and the third drift tube. Theoperation comprises the steps of: (a) providing a gas sample comprisingionic chemical species to the first drift tube; (b) spatially separatingalong the first drift axis one or more of the chemical species bycollisions with a first carrier gas in the first drift tube under afirst set of drift conditions; (c) applying an electrical potentialdifference between the first electrode arranged opposite to the inlet ofthe third drift tube in order to move at least a portion of the ionicchemical species into the second drift tube and substantiallysimultaneously move at least a portion of the ionic chemical speciesinto the third drift tube; (d) spatially separating along the seconddrift axis one or more of the ionic chemical species by collisions witha second carrier gas in the second drift tube under a second set ofdrift conditions and conveying at least a portion of the separated ionicchemical species to an ion detector; and (e) spatially separating alongthe third drift axis one or more of the negative ion chemical species bycollisions with a third carrier gas in the third drift tube under athird set of drift conditions and conveying at least a portion of theseparated ionic chemical species to an ion detector; wherein the secondand third sets of drift conditions are different.

In various aspects, the present inventions provide multi-dimensional ionmobility spectrometric systems comprising three or more dimensions ofIMS. In various embodiments, a MDIMS comprises: (a) an ion source influid communication with a first drift tube, the first drift tube havinga first drift axis; (b) a second drift tube having a second drift axissubstantially perpendicular to the first drift axis; (c) a third drifttube having a third drift axis substantially perpendicular to the seconddrift axis; and (d) a first ion detector in fluid communication with thethird drift tube. In various embodiments, the third drift axis issubstantially perpendicular to both the second drift axis and the firstdrift axis.

In various aspects, the present inventions provide methods of operatingmulti-dimensional IMS systems comprising two or more dimensions of IMS.It is to be understood, for example, that the methods of CFDI can beused with any of the embodiments of a IMS of the present inventionscomprising a first drift tube (dimension). It is to be understood, forexample, that in any of the embodiments of the present inventions thatthe first drift tube (dimension) can be used as one or more of anionization source, reaction region or desolvation region, and driftregion for the system. It is also to be understood that the methods ofthe present inventions can include a step of adding compound to one ormore of the drift dimensions, the compound facilitating the separationof chiral chemical species.

It is believed that various embodiments of the present inventions can bevaluable tools and methods in the detection of trace compounds. By wayof example, and not by way of limitation, it is believed that variousembodiments of the present inventions could provide one of more ofimproved resolution and improved sensitivity in comparison withconventional single dimension IMS systems.

For example with respect to resolution, the resolving power that can beachieved in various embodiments of the multi-dimensional systems of thepresent inventions is predicted to be between about 80 and 100. Comparedto the resolving power of 10-30 offered by conventional commerciallyavailable trace detectors, various embodiments the present inventionscould theoretically in practice resolve 63 more chemicals in an ionmobility spectrum than these conventional IMS systems. For example,assuming in a commercial system a TNT drift time of 10 ms, and a halfheight peak width of 0.5 ms, then the conventional system would have aresolving power (R=t/w1/2) of 20 and peak capacity of 2 peaks/ms. If auseable drift time range in a mobility spectrometer was 9 ms, the systemcould theoretically distinguish 18 compounds in a single mobilityspectrum. In comparison, a system with a resolving power of 90, totalnumber of peaks that theoretically can be distinguished is 81. Such animprovement could theoretically allow the higher resolving system toseparate interferants from targeted explosives; thus, e.g., reducing thefalse alarm rate in contaminated operational environments.

For example with respect to sensitivity, various embodiments of thepresent inventions provide a unique multi-dimensional scheme which canfacilitate improving ion transportation efficiency inside thespectrometer and, consequently, improving system sensitivity. Withimproved resolving power, detection thresholds can be set lower. Invarious embodiments, the system sensitivity of a MDIMS system of thepresent inventions can be in sub-nanogram range under targeted operatingenvironment (not only laboratory conditions).

In various aspects, the present invention provides multi-dimensional IMSbased detection systems in compact size. Instruments of the presentinventions can, in various embodiments, be used as portable tracedetection systems for detection of chemicals, for example, for detectionof explosive materials as may be useful in transpiration security orother uses. Preferably, the new detection systems of the presentinventions have the same or similar operational controls and incorporatea user interface similar to current systems. In various preferredembodiments, a compact high performance trace detection system isprovided which can answer the challenges of performing explosive tracedetection missions in complex environments including maritime/industrialenvironments.

For example, various embodiments of the present inventions can providean MDIMS that offers trace detection in compact size with performancecomparable with or better than certain conventional desktop units, e.g.a Smith 500DT, which is manufactured and available from SmithsDetection. The compact instrument embodiment designs reduce total weightof the system and power consumption. One specific embodiment of acompact design (or hand-held) MDIMS according to the present inventionhas a size that is approximately 12 w×8 h×4 d inches and a weight thatis under 12 pounds and in some embodiments significantly under 10pounds.

Various embodiments of the present inventions include an ion detector. Awide variety of ion detectors are suitable for use in the presentinventions including, but not limited to, to Faraday plates, electronmultipliers, photo-multipliers, charge to photon conversion devices,charge-coupled devices (CCD), etc. It is to be understood that whereveruse is made of an ion detector in the present inventions, an iondetector system can be used instead; where an ion detector system inthis context comprises an ion detector and a mass spectrometer disposedbetween the ion detector and an IMS dimension of the present inventions.Suitable mass spectrometers for this purpose include, but are notlimited to, time-of-flight (TOF) and RF multipole mass spectrometers.

In another aspect, provided are articles of manufacture where thefunctionality of a method of the invention is embedded on acomputer-readable medium, such as, but not limited to, a floppy disk, ahard disk, an optical disk, a magnetic tape, a PROM, an EPROM, CD-ROM,DVD-ROM, or resident in computer or processor memory. The functionalityof the method can be embedded on the computer-readable medium in anynumber of computer readable instructions, or languages such as, forexample; FORTRAN, PASCAL, C, C++, BASIC and, assembly language. Further,the computer-readable instructions can, for example, be written in a,script, macro, or functionally embedded in commercially availablesoftware, (e.g. EXCEL or VISUAL BASIC).

The foregoing and other aspects, embodiments, and features of theinventions can be more fully understood from the following descriptionin conjunction with the accompanying drawings. In the drawings likereference characters generally refer to like features and structuralelements throughout the various figures. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the inventions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B schematically shows cross-sectional views of anembodiment of a three dimensional multi-dimensional ion mobilityspectrometer (MDIMS) device of the present inventions. FIG. 1C showssimulated electrical potential lines within the first and seconddimensions of IMS during a “kick-out” of ions from the first to seconddimension. The second dimension can be used for, for example, singleand/or dual polarity mode operation.

FIG. 2 is an ion mobility spectrum showing the resolution of TNT from4,6-dinitro-o-cresol (4,6 DNOC), a component of acidic fog commonlyfound in airport environments due to jet exhaust. The spectrum wasobtained with a MDIMS with a configuration according to the presentinvention.

FIG. 3 schematically illustrates various concepts of a CFDI process.

FIGS. 4A and 4B are schematic drawings of various embodiments of amulti-dimensional ion mobility spectrometer of the present inventionshaving two perpendicular electric field regions, where FIG. 4A depicts afront cross-sectional view and FIG. 4B depicts a side cross-sectionalview of the MDIMS.

FIGS. 5A-C show a simulation of the electric field distribution in theMDIMS of FIGS. 4A-4B. FIG. 5A depicts drift electric field in the firstand second drift region of both dimensions. FIG. 5B is a sidecross-sectional view of the first dimension electric field duringmobility measurement in the first dimension. FIG. 5C depicts a sidecross-sectional view of the first dimension electric field distributionwhen a “kick out” voltage is applied to bring the ions into the seconddimension. FIG. 6A is schematic drawing of various embodiments of aSDSIMS. In various embodiments, positive and negative ions in the firstdimension can be extracted into two separate drift regions of a seconddimension 603 and positive and negative ions can thus be measuredsubstantially simultaneously. FIGS. 6B and 6C are simulation results ofelectric fields distribution in the first dimension 622 and 624 and thesecond dimension 620 a and 620 b. FIG. 6B depicts the electrical fields622 before and/or after a “kick out” pulse is applied. FIG. 6C depictsthe fields 624 during the application of a “kick out” pulse.

FIG. 7 is schematic drawing of various embodiments of a DSDIMS withmultiple second dimensions in parallel position and multiple ionizationsources.

FIG. 8 is schematic drawing of various embodiments of a DSDIMS withmultiple dimensions in an opposing position.

FIG. 9 is schematic drawing of various embodiments of a DSDIMS with asingle sample source and multiple first dimension chambers; which can beused, for example, with different polarities.

FIG. 10 is schematic drawing of various embodiments of a MDIMS forsampling chemicals in ionic form and/or from an external ionizationsource.

FIGS. 11A and 11B are schematic drawing of various embodiments of aMDIMS useful, for example, for SII and MS^(n) implementation.

FIGS. 12A-12C illustrative various embodiments of a one MDMSconfiguration choice for a portable three dimensional instrumentaccording to various embodiments of the present inventions. FIGS. 12Aand 12B provide schematic two-dimensional cross sectional views and FIG.12C provides a schematic three-dimensional cross-sectional view.

FIGS. 13A and 13B are schematic scale drawings of the MDIMS system ofFIGS. 14A-14B.

FIGS. 14A-14B are scale schematic drawings of a preferred embodiment ofa portable MDIMS incorporating various embodiments of the presentinventions.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In various aspects, the present invention provides multi-dimensional ionmobility spectrometry (MDIMS) systems, preferably with multi-dimensionalelectric field designs in one integrated spectrometer, and methods ofoperating such systems. In various embodiments, the MDIMS systems and/ormethods provide improved sensitivity and resolution compared toconventional single dimension drift tubes. In various embodiments,improved sensitivity can be achieved by using the first dimension as anion storage region to improve system duty cycle. In various embodimentsthe MDIMS systems and/or methods provide improved mobility resolution.In various embodiments, improvements can be achieved by the use of driftregions which can further separate ions that are or have already beenseparated based on their mobilities. In various embodiments, as ionspecies are being separated in the first dimension, the columbicrepulsion among them is reduced by transferring them to a second IMSdimension (e.g., using a kickout pulse). Thus, in various embodiments,higher mobility resolution can be experienced in the second dimension.In various embodiments, the first dimension can be used as an ionreaction region where further ion conversion can be achieved. In variousembodiments of a MDIMS, and appropriate electric field application, aMDIMS can be used to detect both positive and negative ionssubstantially simultaneously.

Prior to further describing various detailed embodiments of the presentinventions, it may be helpful to a fuller understanding thereof todiscuss various embodiments of the apparatus and methods of the presentinventions in the context of one embodiment of a three dimensional MDIMSdevice.

FIGS. 1A-1C illustrate various embodiments of a three dimensional MDIMSsystem. FIG. 1A is a side view of a first dimension drift region 102 anda second dimension drift region 104. FIG. 1B shows a side-view of thesecond dimension drift region 104 and a third dimension drift region106. In FIGS. 1A-1C, the second dimension comprises two drift tubes 104a, 104 b, and there is a separate third dimension drift tube 106 a, 106b associated with each of the second dimension drift tubes. The seconddimension 104 can be used for single or dual polarity mode operation. Invarious embodiments of the MDIMS, it is understood that a preferredembodiment is to arrange the drift axis of each dimension in orthogonalgeometry, however, the drift axis can be arranged in parallel,anti-parallel or with an angle in between to achieve similar results.

It is to be understood, that the electrical drift field strength-to-gasnumber density ratio (E/N value, often expressed in units of Townsend)in all IMS dimensions of the present MDIMS apparatus and methods ischosen to establish a steady-state drift environment, sometimes referredto as a low field environment.

With the MDIMS of the present inventions, the ion mobility spectrum canbe represented, e.g., in a 2-D or 3-D plot, and can use a non-lineardetection window. Chemicals can be identified in their 1-D, 2-D or 3-Dmobility profile. This mobility profiling method can provide additionalinformation and thus, can provide greater confidence for chemical (e.g.,explosive) identification..

In various embodiments a DPIE operational mode can be conducted usingthe first dimension 102 as a flow through cell where both positive andnegative ions are brought into the first drift chamber by gas flow whilethe drift voltage in the first dimension is turned off (i.e.,substantially no drift field is present). At a predetermined time ionsare and kicked out into the second dimension, preferably such that thepositive and negative ions in the first dimension are substantiallysimultaneously extracted into two separated drift chambers 104 a, 104 bin the second dimension 104. After ions are separated in the seconddimension 104, they can be further separated and detected in the third106 or higher dimensions.

In various embodiments, ionized samples are guided into and/or formed inthe first drift region 102 and subject to a first order separation basedon mobility (resembling conventional IMS). At a given predeterminedtime, separated ions in the first dimension (first drift tube) arekicked out into the second drift dimension 104 drift region where theyare separated in the direction that is substantially perpendicular tothe first drift direction. The same process can be continued in thehigher dimensions if desired with further dimensions of IMS.

FIG. 1C shows simulation results of the electric field distribution of aDPIE process in a DSDIMS of FIGS. 1A and 1B. In FIG. 1C, the three wallsin the first dimension 102 (left, bottom, and right) are at 1,000 V andthe gate grids are set at 0 V and 2,000 V respectively. Theequi-potential lines are shown in the figure. The sample gas flow usedto carry ions through the first dimension can be exhaust, e.g., behindthe first dimension detector 114. After ions are separated in the seconddimension 104, a kick out voltage can be applied to bring the separatedions into the third dimension 106. In a continuous sample detectionscenario, the sequence will repeat. For a chemical mixture that may formboth positive and negative ions, various embodiments of the DPIEtechnique can extract more than 50% of both positive and negative ionsinto the second dimension.

In various embodiments, the MDIMS devices can transport ions betweeneach dimension without significantly losing resolving power. Referringto FIG. 11, in various embodiments, when ions are separated in the firstdimension; they can look like a thin plate 1110. To move them into thedirection that is perpendicular to the first dimension, voltages arechanged on the appropriate electrodes (typically an electrode oppositethe inlet, the inlet itself, or both) within a microsecond range. Theelectric field during these kick out moments can be manipulated tocreate temporary high and low electric field zones. The thin plate 1110in the high field zone can be compressed into a thin line 1120 in thelow field zone of the second dimension.

One area of application of the present inventions is in the detection oftrace amounts of chemicals, such as is often required in securityapplications, such as drug and explosive screening. In practice suchtasks can be difficult to perform for a variety of reasons.

For example, detecting trace explosives in a highly contaminatedenvironment poses great challenges to current IMS-based trace detectionsystems. The contamination can either cause false positive or falsenegative indications. A few common phenomena observed in commerciallyavailable IMS systems that can lead to these problems are:

1. Overlapping or adjacent peaks in the explosive detection window fromchemicals that have similar ion mobilities as targeted explosives, thatcan cause false positives;

2. Undefined ion mobility peak shifting through explosives detectionwindow that can cause false positives; and

3. High chemical background noise from contaminants can cause falsenegatives of explosive detection;

Various embodiments of the systems and methods of the present inventioncan facilitate overcoming these problems in trace chemical detection.

The first phenomenon mentioned above (overlapping or adjacent peaks) canbe reduced using MDIMS in accordance with various embodiments of thepresent inventions. Examples of the performance of a high resolution IMSis illustrated in FIG. 2. FIG. 2 shows ion mobility spectra of TNT and4,6-dinitro-o-cresol (4,6 DNOC). The compound 4,6-dinitro-o-cresol (4,6DNOC) is a component of acidic fog commonly found in airportenvironments due to jet exhaust. It can be seen that even though TNT and4,6 DNOC have very similar ion mobilities of 1.59 and 1.54,respectively, with the resolving power of about 60, they are separatedand properly identified. In the MDIMS system, two high resolution driftchambers as shown in above example can be used to generate atwo-dimensional mobility profile of both positive and negative ionssimultaneously. The two-dimensional ion mobility data provides higherconfidence in explosive detection. As a practical operational approach,first dimension mobility spectra can be acquired for higher throughputscreening; when peaks are detected in the explosive detection windowfrom the first drift chamber they are then brought into the second driftchamber for confirmation.

The second phenomenon mentioned above (undefined ion mobility peakshifting through explosives detection window causing false positives)can be caused by clusters of ions and neutral molecules in the driftregion. In conventional IMS, counter current drift flow designs havebeen used to reduce this effect, however, because of the spacelimitations in conventional IMS designs, neutral reactive molecules arenot completely removed from the drift region. In a one method ofoperation of a MDIMS according to the present invention, the drift flowis set such that substantially no un-ionized sample is introduced intothe drift chambers.

The third phenomenon (high chemical background noise from contaminantscausing false negatives in explosive detection) represents anotherfundamental issue with current IMS-based detection systems. It can becaused by competition for charges in the ionization source; for example,interferants can reduce ionization efficiency of the sample andtherefore reduce detection sensitivity. With current IMS-based tracedetection systems, the thresholds of explosive detection windows are setwith consideration of this masking effect. With higher resolution ofvarious embodiments of the MDIMS systems and methods of operation of thepresent invention, the alarm thresholds can be set to a lower level.

In various embodiments of the present invention, a CFDI mode ofoperation of the MDIMS is used to achieve pre-separation of chemicalspecies that have different charge affinities in a sub-second timeframe. Accordingly, in acquired 2-D ion mobility profile,interferants/masking agents with charge affinities that are differentfrom explosives will locate at different positions in the profile, andsuch becomes part of the differentiation and identification process.

Further Details of MDIMS Apparatus and Methods of Operation

Referring to FIG. 4, provided is a schematic drawings of variousembodiments of a multi-dimensional ion mobility spectrometer having dualsecond ion mobility dimensions. FIG. 4A is a front cross section viewand FIG. 4B is a side cross sectional view of the MDIMS.

In various embodiments, a MDIMS comprises an ionization source 401 to,for example, (a) generate reactant ions and a reaction region wherereactant ions can react with samples and form product ions to bedetected for sample identification; (b) generate sample ions fordetection, (c) or both. The reaction region can be guarded by ion guides402 that generate a substantially continuous electric field to, e.g.,lead the ions to the first dimension drift region 417 (first drifttube).

Multiple Step Separation (MSS) Mode

In MSS mode operation, a pulse of ions are generated by opening an iongate 403, to introduce them into the first dimension drift region 417;the ions are separated based on their mobilities under the guidance asubstantially continuous electric drift field in the first drift tube417. In one embodiment, the electric field is generated by a series ofion guides 404. Each ion guide can comprise one or more electrodes; anddifferent voltages can be applied on each electrode to establish thepotential difference across the first drift tube. For example, FIG. 4Bshows four electrodes used for each first dimension ion guides.

In various embodiments of MSS mode operation, as a first group of ionsreaches the first dimension detector matrix 405, a kick out voltage canbe applied to generate a high electric field that is perpendicular tothe first dimension drift field, thus the ions separated in the firstdimension are moved into the second dimension drift region 418. Anelectric field separator screen 406 can be used to help define theelectric field in the second dimension. Ions introduced into the secondfield will continue to drift across the second dimension drift region418 and further separation can be achieved. The ion guides 407 in thesecond dimension 418 can be arranged similarly to the first dimensionion guides 404, for example, if a third dimension of separation isdesired. If a third dimension is desired, complete square electrodes canbe used as the ion guides. Ions separated in the second dimension can bedetected by the detector 408. The detector can comprise multipledetectors according to required special resolution of the spectrometeror a single detector.

In various embodiments, a partial kick out operation can be performedwhen ions are introduced from the first dimension to the seconddimension. If only a portion of the ions are kicked out, the mobilitymeasurement in the first dimension can be resumed after the kick out.Thus, an ion mobility spectrum can also be acquired independently in thefirst dimension. As a complete kick out can increase the sensitivity inthe second dimension, alternating between these operation methods can bebeneficial. In addition, a clean up operation, e.g., remove all ions inthe drift chambers by an applied “kick out” electric field for anextended period of time, can also be added between detection cycles.

The low dimension operation of the spectrometer can be used as fastscreening method to generate a quick survey of the ionic species fromthe ionization source. In combination with the normal operation of theMSS mode, the survey of the ionic species can be used as an index toguide upper dimension operations. The survey mode operation can also beused to selectively kick out ions of interests, simplify higherdimension spectra, and save total analysis.

Different drift/separation conditions can be established independentlyfor each dimension, e.g., different drift gases may be used in eachdimension or different drift gas temperatures in each dimension.

The MDIMS can be operated in a fashion where a number of multipledimensional positive ion mobility data is collected followed by a numberof multiple dimensional negative ion mobility data. The sequence can berealized, e.g., by alternation the polarities of electric fields in thespectrometer.

FIG. 5 shows the simulation results of electric field arrangement insidea MDIMS substantially similar to that of FIGS. 4A-4B. FIG. 5A shows twoperpendicular electric fields 502 and 504 can be arranged in the MDIMS.In FIG. 5B and 5C, two half square ion guides 508 and 510 are simulatedas an example. FIG. 5B shows side view of first dimension drift region.While ions are drifting down in this region, both half-square electrodes508 and 510 are set at the same voltage; thus, there is substantially noelectric field perpendicular to the drift field. Referring to FIG. 5Cwhen ions in the first dimension are kicked out into the seconddimension, different voltages are applied on the half square ion guides508 and 510 creating a kick out field 506. In this particularsimulation, the upper half square ion guides 510 are at 1,000 V lowerpotential than the lower half-square ion guides 508. The kick outoperation can be achieved, for example, in the microsecond to secondsrange.

During MSS mode operation, the directions of the drift gas flow can beset to be counter to or across from the ion movement. For example, invarious embodiments, gas port 413 can be used as the second dimensiondrift gas inlet and port 412 as the first dimension drift gas inlet,port 409 as the sample flow inlet and port 410 as purge gas outlet. Theother ports are preferably plugged or remove when they are not in use.The size of each port can be selected depending on the flow required toachieve the flow pattern inside the spectrometer and preferably thedrift flow sweeps the entire drift region and removes excessive samplemolecules and any other reactive neutral molecules.

In various embodiments, the drift gas can be supplied to the higherdimension in the direction that is in substantially parallel to thelower dimension. For example, port 415 can be used as the seconddimension drift gas inlet, port 414 as the second dimension drift gasoutlet, port 412 as first dimension drift gas inlet, and port 410 as thefirst dimension drift gas outlet. Under linear flow conditions and theparallel flow pattern, for example, limited mixing of drift gas near thedimension interface is expected.

Storage and Burst Analysis (SBA) Mode

In SBA mode operation, the sample is provided into the spectrometerthrough port 409. Through the ionization source 401, the ionized thesamples are brought into the first dimension drift region 418 by gasflow. In case where only a single polarity of ions is of interests, theflow can be purged from port 412 or port 411. In various embodiments ofthe SBA operational mode, the first dimension drift tube can be used asion storage device to, e.g., increase the duty cycle of the device.

In various embodiments, where both positive and negative ions are ofinterest, a Dual Polarity Ion Extraction (DPIE) method can be used.FIGS. 5A and 5B shows the electric field generated during ion storageand DPIE operation. For example, FIG. 5C shows that three walls in thefirst dimension (left, bottom, and right) are at 1000 V and gate gridare set at 0 V and 2000 V, respectively. The electric field distributionshown in FIG. 5B illustrates that the gas flow is used as the force tocarry ions through the first dimension 502, where the electric field inthe first region 418, 502 is set to substantially zero until a kick outpulse is generated. In the case of using port 412 as the sample flowexit, when electric fields in both the reaction region 416 and the firstdrift region 418 are removed; the sample ions will only be carriedacross the first dimension by gas flow. In various embodiments, when thedetector 405 detects a sufficient ion current level, a complete kick outtoward the second dimension 420, 504 is be performed. In a continuoussample source detection scenario, the sequence is repeated.

Selective Higher Dimension Ion Monitoring Mode

In various embodiments, selectively monitoring ion current at a specificelectrode of higher dimension detector matrix can improve systemselectivity by eliminating uninterested ions on other electrodes in thesame detector matrix. The ion mobility profile can be constructed usingthis selective monitoring method with signals from a plurality ofelectrodes. In various embodiments, selectively monitoring ion currentat a specific electrode of higher dimension detector matrix can improvesystem selectivity by eliminating uninterested ions on other electrodesin the same detector matrix.

In various embodiments, of the MDIMS, the higher dimension drift chambermay have a reduced length. In these embodiments, the device issimplified. The ion mobility based separation achieved in the lowerdimension and detected on the higher dimension detector matrix withfurther separation in the higher dimensions. For example, ion can beseparated in first and second dimensions, and then they are detected onthird dimension detector matrix without further separation in the thirddimension. In this case, the “kick out” timing is controlled to moveions into higher dimension with optimal ion mobility resolution and ionpopulation to maximize the system performance

Continuous First Dimension Ionization (CFDI) Mode

In various embodiments of CFDI mode operation, the samples areintroduced to the spectrometer from port 412 as pulses of gas. Thesample gas pulse can be formed in a wide variety of ways, for example,by thermally desorbing chemicals from a surface, as the eluent of achromatographic separation, by pumping the sample into the spectrometerfor a short period of time, introduction through a pulsed valve, etc. Inmany embodiments, the flow under a linear flow condition, and a “plug”of gas phase sample is directed from the port 412 towards the ionizationsource 401 by gas flow. Pulses of reactant ions (preferably at highdensity) are generated by the ionization source 401 and guided by theelectrical drift field to drift towards the sample “plug”. As the pulseof reactant ions and samples intercept in the first dimension 417, aportion of the samples are ionized. As the sample encounters multiplereactant ion pulses in the same acquisition period, chemicals in thesample “plug” are ionized. Chemicals with different properties (e.g.,charge affinity) can thus be separated and detected at differentlocations on the detector matrix 408. This gas phase titration methodcan improve ionization efficiency of ion mixture where chemicals withdifferent properties coexist. By this means chemicals that can not bedetected in conventional IMS can be detected.

Referring to FIGS. 3A-D, a schematic representation of the CFDI processis illustrated in a drift region 302. A gas phase sample 304 is pulsedinto a drift tube 302 at a first time and conveyed in a first direction305 along at least a portion of the drift tube, wherein the firstdirection is substantially parallel to the direction of carrier gas flowin the drift tube. The speed of the carrier gas flow and gas phasesample is equal or greater than zero cm/second. A plurality of pulses ofreactant ions 306 are also introduced into the drift tube 302 atpredetermined times relative to the first time and conveying by theelectrical drift field in a second direction 307 along the drift axis308, wherein the second direction 307 is substantially anti-parallel tothe direction of carrier gas flow 305 in the drift tube. The gas sample304 interacts with a first group of reactant ions 310 (comprising one ormore of the plurality of pulses of reactant ions) to ionize a chemicalspecies in the gas phase sample pulse 304 and produce a first ionizedchemical species 314. In various embodiments, a kick out field isapplied (e.g., by application of a kick out voltage to an electrode set318, 320 and 322) to move the ions 314 in a direction 316 out of thedrift tube and into another drift dimension. In various embodiments, theprocess repeats for other groups of ions 311, 312, that interact withthe gas sample 304 to produced further ionic chemical species.

In various embodiments, the CFDI can also be performed in the reactionregion 416, shown in FIG. 4. A plurality of pulses of reactant ion isgenerated by pulsing ion gate 419 while pulsed sample are introduce tothe spectrometer from gas port 410. In this implementation, ion gate 403is removed or kept open. Pulse of ions generated in the reaction region416 are separated in first dimension drift region 417, and then theseparated ions are extracted in higher dimension drift region 418 forfurther ion mobility analysis if so desired. In various embodiments, theCFDI method can be used as an independent ionization source directlyinterfaced to spectrometers, such as differential mobility spectrometer,ion mobility spectrometer or a mass spectrometer, either inline orperpendicular to the direction drift electric field. In embodimentswhere CFDI is used for a single IMS, the shutter grid 419 will be usedinstead of grid 403. The ionized chemical species continue to drift indrift region 417 after formation in the reaction region 416. Similarly,interfaces to other spectrometers, such as differential ion mobilityspectrometers and mass spectrometers, can also be realized by placingthe sample inlet of these instruments directly after the reactionregion.

The CFDI mode can be preformed using reactant ions with differentchemical properties. For example, modifying the ion chemistry using avariety of chemical reagents that react with initial reactant ions cangenerates reactant ions with different chemical properties. These ionicspecies can be used, e.g., to ionize samples introduced to thespectrometer. Similar effects can be achieved, e.g., by using anionization source that can generate different ionic species or chargedparticles/droplets. In various embodiments, altering the ionizationchemistry can be used to achieve substantially selective ionization oftargeted chemicals in the sample. For example, a series of ion pulsewith different chemical properties can be used to ionize chemicals withcompatible ionization properties in the sample.

Selective Ion Introduction (SII) and IMS^(n) modes

In Selective Ion Introduction (SII) mode operation, one or multiplegroups of selected ions are kicked out into a higher dimension. Theselective kick out can be realized by applying a kick out voltage at apredetermined time to the region where ions of interests are travelingthrough at a given timing. In various embodiments, the kick out pulse isnot necessarily applied to a selected region of the lower dimension, butthe higher dimension drift chamber does not intercept the lowerdimension only over a portion of length of the lower dimension; thus,e.g., a selected location can be designed only to allow a small group ofions to be kick out into the second dimension. A similar result asdescribed with respect to MSS mode can be achieved by controlling thekick out timing and performing multiple acquisition cycles.

In various embodiments, the SII mode can be effective in resolving ionsin a narrow drift time range. For example, suppose a first driftdimension is used as a screen scan, and a compound of interest (e.g.,TNT) is detected as potentially present. To further confirm that ionresponded in the detection window (time window) is the compound ofinterest, one can selectively kick out the peak in that detection widowinto the second dimension for further separation. From the seconddimension, ions that fall into a selected window can be kicked out intoa third dimension. This process can be repeated until the ion current isexhausted if so desired.

Multiple Drift Chamber Condition

In the various methods and operational modes, each drift chamber isoperated under independent and/or different drift conditions. Theseconditions include, but are not limited to, different kinds of driftgases, drift gases with different chemical modifiers, differenttemperatures, different pressures, different electric field strength,different flow rate, different phases of drift media, and directions,etc. In various embodiments, the purpose of changing the conditions isto achieve separations of the ionic species using their unique chemicaland/or physical properties and how these can change with drift conditionand thus can result in mobility changes in the spectrometer. Forexample, ion mobility measurements using different drift gas havedemonstrated that ions with different properties can have differentdrift time in the sample spectrometer (See, for example (1) William F.Siems, Ching Wu, Edward E. Tarver, and Herbert H. Hill, Jr., P. R.Larsen and D. G. McMinn, “Measuring the Resolving Power of Ion MobilitySpectrometers”, Analytical Chemistry, 66, 1994, 4195-4201; (2) Ching Wu,William F. Siems, G. Reid Asbury and Herbert H. Hill, Jr., “ElectrosprayIonization High Resolution Ion Mobility Spectrometry/Mass Spectrometry”,Analytical Chemistry, 70, 1998, 4929-4938; (3) Ching Wu, Wes E. Steiner,Pete S. Tornatore, Laura M. Matz, William F. Siems, David A. Atkinsonand Herbert H. Hill, Jr., “Construction and Characterization of aHigh-Flow, High-Resolution Ion Mobility Spectrometer for Detection ofExplosives after Personnel Portal Sampling” Talanta, 57, 2002, 123-134;and (4) G. Reid Asbury and Herbert H. Hill, “Using Different Drift Gasesto Change Separation Factors (α) in Ion Mobility Spectrometry”,Analytical Chemistry, 72, 2000, 580-584; the entire contents of all ofwhich are hereby incorporated by reference).

In various embodiments of MDIMS systems, the higher dimension driftregion, such as the second dimension region, can be operated indifferent phases of drift media, e.g. gas or liquid. The liquid phasedrift cell can be constructed with two parallel plates or grids insteadof a conventional drift tube design. The liquid phase drift cell can bea thin layer of liquid that has an electric field across the layer. Thehigher order dimension drift cell has drift axis that is substantiallyparallel or substantially perpendicular to the first dimension driftaxis. The higher dimension drift cell has multiple compartments(channels) that are substantially perpendicular to the lower dimensiondrift axis. The higher dimension drift cell can be used for selectivelycollecting samples separated in the lower dimension drift tube. Thehigher dimension drift cell can be further interface to other separationand detection apparatus, including but not limited to electrophoresis,chromatography, UV absorption and other spectroscopic apparatus.

In various embodiments of MDIMS systems of the present inventions,different drift gases are used in different drift tubes and/ordimensions of the MDIMS to separate ionic species in a higher dimension(e.g., a second dimension) that are not sufficiently separated in thedrift gas in a lower dimension (e.g., the first dimension). It is to beunderstood that the drift gas can be a mixture of two or more gases.Similar separations can also be done by varying other drift chamberconditions.

Further Examples of MDIMS Configurations

FIG. 7 shows various embodiments of a MDIMS where multiple higherdimensions drift chambers 704 a, 704 b are arranged in substantiallyparallel and multiple ionization sources 706 a, 706 b are used, forexample, to generate ions in both positive and negative polarity. Forexample using a the CFDI mode of operation, a sample can be introducedto the spectrometer from the port 708 b in the center of the firstdimension; two ionization sources of different polarity can be used togenerate high density reactant ions that are guided into the firstdimension chamber 702 a, 702 b by an electric field that moves thereactant ions toward the center of the first dimension. Where anelectrospray ionization source is used, for example, charged dropletscan be used for ionizing the sample using the secondary electrosprayionization principle. The ionized chemicals are brought into the higherdimensions 704 a, 704 b for mobility measurements. This embodiment canbe used, for example, to analyze the same sample using differentionization sources using different ionization modes and'/or driftconditions as described above for example. For example, the device canhave more than two first dimension reaction chambers and higherdimension drift chamber combinations to utilize more ionization methods.The higher dimensions can be operated in single or dual polarity mode(e.g., DPIE) to extract ions from the first dimension.

FIG. 8 is a schematic drawing of various embodiments of a MDIMS with ahigher dimension 804 a, 804 b extending in opposite directions from alower dimension 802. The second dimensions can be operated in single ordual polarity mode as previously described. The first dimension can beoperated, for example, as an ion flow cell for ion storage. Thisconfiguration can be utilized, for example, such that ions withdifferent polarities can be kicked out into the opposite higherdimension drift chambers when it is operated under a SBA mode. Invarious embodiments where the higher dimension drift chambers are dualmode chambers, the DPIE method can be used, e.g., to deliver ions toboth dual mode chambers for independent analysis where the dual modedrift chambers can be operated under different drift conditions.

FIG. 9 is a schematic drawing of various embodiments of a MDIMS withionization source 906 and sample inlet 908 between the inlets to thedrift tubes of a second dimension 909 a, 909 b. Ions formed in thisionization source 906, e.g., can be extracted into two differentsections of the first dimension 902 a, 902 b. Each section can beoperated in either positive or negative polarity mode. For example, invarious embodiments each section of the first dimension of the MDIMS isused as a first dimension drift chamber. Each section of the firstdimension can have its own higher dimension drift chamber for furtherion separation.

FIG. 10 schematically depicts various embodiments of an MDIMS of thepresent inventions for sampling chemicals in ionic form or from anexternal ionization source. Such embodiments include a source 1004 influid communication with a first drift tube 1002 through an interface1006. The methods of the present inventions that operate on ions andoperational modes described herein can used with such embodiments. Thisembodiment eliminates the necessity of internal ionization source andreaction region of the IMS system. The ionized chemicals are eitherbrought into the interface 1006 by an electric field (in this example,the ionization source 1004 and interface 1006 are set at differentpotentials), or by gas flow (in this example, the ionic species 1008 aremoved into interface 1006 by sampling pump 1009. Once the sample ions1008 are moved in to the interface 1006, they are pulsed into the firstdrift dimension 1002 through ion gate 1003. They are either detected onfirst ion detector 1012 or subsequently kicked out into the seconddimension drift region 1010 and detected on detectors 1014, underguidance of ion guides 1005 and 1011, respectively.

IMS^(n) and Hyphenate Systems

FIGS. 11A and 11B show schematic examples of various embodiments thatcan be used to realize the SII mode operation with IMSn. By reducing thephysical size of the higher dimensions and controlling the timing of thekick out pulse, a selected group of ions 1114 that drifted into the kickout region 1112 can be brought into a higher dimension drift chamber1118 where they can be further separated. The same process can becontinued until the nth separation performed in different driftchambers. The geometry of the interconnected drift chambers can be twodimensional (FIG. 11B) or three dimensional (FIG. 11A), thus the numberof times a higher order mobility separation can be conducted is notnecessarily limited by the physical space available for thespectrometer.

In various embodiments, FIG. 11A shows schematic of a three dimensionalMDIMS that illustrate SII mode operation. When gas phase sample isintroduced into the reaction region of the first dimension drift tube,between ion gates 1103 and 1108, the sample is ionized by either CFDI orconventional ionization methods with reactant ions created by ionizationsource 1102. The sample ions mixed with reactant ions are pulsed intothe first drift region 1104. Under the guidance of the electric fieldgenerated by ion guide 1106, the ion mixture separates in the firstdimension. At a predetermined timing when ions of interest 1114 driftinto the kick out region 1112, a kick out voltage can be applied to aset of electrodes (including spited ion guide 1116 and grids 1130) toextract ions into the second dimension. As ions 1114 are compressed inthe interface between the kick out region 1112 and second dimensiondrift region 1118, narrow pulses of plural separated ions 1120 arecreated at the beginning of second dimension drift region 1118. The ionspulses 1120 are separated in the drift region 1118 that is guarded byion guides 1119. The further separated ions 1124 are extracted fromsecond ion kick out region 1122 into the third drift chamber that has adrift direction 1126 (pointing insider the paper) that is orthogonal tothe first and second dimension. The extracted ions repeat the processdescribed above in the third dimension or higher.

In various embodiments, FIG. 11B shows schematic a MDIMS operating inSII mode with a two dimensional structure. FIG. 11B illustrates that onepeak 1114 b isolated by the first dimension drift tube is extracted intosecond dimension, and then one peak isolated by the second dimensiondrift tube is extracted into the third dimension 1132 having a driftdirection that is substantially perpendicular to the second dimensionand substantially anti-parallel to the first dimension. In this example,the drift axes of all dimensions are on the same plane.

For example, in various embodiments, the configuration of FIGS. 11A and11B can be interfaced to other detectors, such as a mass spectrometer.IMS-MS systems are commonly used to achieve mobility based separationbefore mass analysis. The interface to a mass spectrometer can bein-line with ion drifting direction behind the detector matrix, e.g.1122 or 1136. FIG. 11B shows an interface to a mass spectrometer 1128 athrough an opening on the second dimension detector matrix 1138, orperpendicular to the drifting direction using a kick out pulse to pushions into the interface 1128 b and 1128 c. Higher ion transportationefficiency is expected in the later case.

Sampling Apparatus

Samples can be introduced to the MDIMS either as a pulse in the flowstream, continuously, or combinations thereof. The pulsed samplingmethod can be used in several operational modes. A thermal desorptionchamber in the front of the spectrometer can be used to provide samples.For example, for samples from a swab a desorption chamber cancontinuously heat up the swab and then pump high concentration chemicalvapor into the MDIMS. A valve can be used to control the amount ofsample allowed to enter the spectrometer. Depending on the operationalmodes described above, sample may be allowed to continuously flow intothe MDIMS with an ionization source. The desorption chamber can containadsorbent materials for a sample pre-concentration step of operation.For example, as a low concentration or complex mixture sample isintroduced to the chamber, adsorbent can selectively trap compounds ofinterests and then desorb them into the MDIMS.

Sample Swabs

There are numerous ways to improve sampling efficiency. In variousembodiments, “wet” sample swabs can be used to facilitate completesample collection instead of dry swabs. The wet swabs can assist thecollection efficiency by increasing the contact between swab andsurface, and provide better physical pick-up of sample. By selecting anappropriate solvent mixture, the targeted explosive can also dissolveinto the swab, thus achieving higher sampling efficiency. To facilitatewet swabbing operation a matching sample swab and desorber design can beused.

The swab is preferably designed with a pattern of hydrophobic andhydrophilic surfaces for the designated solvent or solvents to be used.This pattern can match, e.g., the pattern of the heater inside of thedesorber. In a practical search scenario, the dry and wet sample swabsare often used depending upon the sampling surface. Since the wet swabhas higher collection efficiency, it could be used, e.g., forconfirmative tests to resolve an alarm from a dry swab; it could be usedin a “complete wipe operation” to collect low level explosivesparticles, etc. Since the explosives can be soaked into or adhere to thesample swab, it could also be used to reserve samples as evidence

Air Sampling

In various embodiments, the MDIMS systems of the present inventions canbe operated with continuous sampling of vapor in surroundingenvironments. The sample frequency can be preset to serve the purposefor either early warning of, e.g., a high quantity of explosives orother safety concerns. Common volatile explosives, such asnitroglycerine, TATP or even explosive Taggants, can be detected fromthe vapor phase. Thus, in various embodiments the MDIMS systems of thepresent inventions can be used for “sniffing”.

Sample Concentration

In various embodiments, the sampling capability and/or desorptionefficiency can be enhanced by using a pattern of high chemical affinitycompounds, chemical resistive surfaces, or both on the inner wall of thedesorption chamber. The high chemical affinity compound coated surfacecan be used to preconcentrate vapor in the gas phase and/or selectivelytrapping target compounds in the presence of other chemicals, such assolvents used for wet sampling. The heating element for the desorbedparts can be arranged, e.g., to heat up the area where the high chemicalaffinity coat is applied. Such a selective heating approach can be used,e.g., to reduce the heat used for desorption, and thus reduced the totalpower consumption of the detection system.

Sample Ionization

The MDIMS systems of the present invention can comprise one or moreionization sources. The ionization sources can be used to generatereactant ions, directly ionize targeted chemicals, or both. Suitablesources include, but not limited to, radioactive ionization,electrospray ionization, desorption electrospray ionization, surfaceionization, and corona discharge ionization sources.

Electrospray ionization is one of the preferred sources for inorganicexplosives detection. ESI-MDIMS can be used, e.g., to detect chloridebased explosives, as well as black powers with high confidence. With awet sampling scheme, e.g., electrospray ionization can be used toprocess the wet samples by directly spraying collected sample into theMDIMS. One implementation of this method includes having the wet samplesput into a sample holder, which has an electrospray needle andelectrodes where an electrospray voltage can be applied. As the sampleis sealed inside the holder, pressure is applied to the holder/soakedsample swab either directly or indirectly, and the solvents anddissolved sample reach the electrospray needle, and are electrosprayedto form highly charged droplets. The electrospray sample ions can beguided into the MDIMS for analysis. The combination of wet sampling anddirect electrospray ionization for the MDIMS can provide, for example,detection capabilities for both inorganic and organic explosives andother chemicals of interest.

Increasing Detection System Usability

Improved system readiness. Although existing IMS-based trace detectionsystems can typically meet the throughput requirements in an airportoperational environment in undemanding situations, the sample throughputis limited when highly contaminated samples are introduced to thesystem. Accumulation of contaminants in the system can require long bakeout times and/or a complex cleaning procedure using organic solvents.Besides cold spots and active surfaces inside the sample transferportions of the system and detector, a membrane inlet (e.g., as found inboth Sabre 4000 and VaporTracer instruments, manufactured and availablefrom Smiths Detection and GE Security, respectively) used to block lowmolecular weight contaminants and moisture is one of the most commonparts that accumulate higher molecular weight contaminants. To eliminatethis main source of “memory effect”, various embodiments of a MDIMS ofthe present inventions use a pulsed inlet system.

In various embodiments of the pulsed inlet, the detector is only exposedto the outside world for a short period of time, typically less thanabout 20 seconds. The valve only opens when the sample reaches thehighest concentration in the sampling chamber. The chamber can be flashheated periodically to clean up accumulated chemicals. The pulsed sampleinlet operating scheme is compatible with various embodiments of theMDIMS systems of the present inventions and can help control the totalamount of moisture introduced into the drift chambers. A humidity sensorcan also be made part of the system to provide additional calibrationinformation for system processing.

One of the most common user errors in conventional systems ismiscalibration. Preferred systems of the invention offer aself-calibration/self-diagnostic algorithm that can calibrate the systemwhen it is powered on and periodically without requiring operatorattention. In various embodiments, the calibrant preferably lasts thelife time of the instrument. This feature can further improve systemreadiness.

The apparatus of the present invention can be constructed as highlyportable instruments. As in any analytical device, there is often atradeoff between the number of operational features and systemportability. In various embodiments of the MDIMS designs and operationalmethods of the present inventions, the total power consumption anddetector size can be reduced. A relatively high fraction of power can beallocated to usage to the front end, including sample desorption andeffective clean up operations. Low power consumption computer basedsystems, e.g., on modern PDA design or the like are preferably used asan element of the portable system. In the balance of selecting betweenportability and usable features, various embodiments of the presentinventions provide a MDIMS detection system with reasonable size that isunder ten pounds, and possibly even under eight pounds.

In various embodiments of the MDIMS, FIG. 12A-C shows the schematic ofan example of the compact MDIMS. The device is configured with threedimensions, including one first dimension chamber 1202, two seconddimension drift chamber 1204 a, 1204 b, and two third dimension chambers1206 a and 1206 b, with a largest dimension of <10 cm. FIG. 12C is thethree dimensional drawing of the MDIMS to the scales. The configurationis to realize both CFDI and DPIE with SII mode. In CFDI operation, thereactant ions are formed in ion source 1210 and pulsed into the reactionregion 1208 to selectively ionize pulsed sample 1212. Ionized samplesare separated in first dimension drift region 1202 and then furtherseparated in higher dimension drift region 1204 and 1206.

In DPIE operation, both positive and negative ions formed in theionization source 1210 and reaction region 1208 are carried into thefirst dimension 1202 by carrier flow without effluence of the electricfield. The positive and negative ions are extracted in to the seconddimension drift chambers 1204 a and 1204 b, respectively. The sampleions are detected on the detector matrix in the first dimension 1214,second dimension 1216 or third dimension 1218 a and 1218 b depending onthe instrument usage and it is software controlled. For fast screeningoperation, ions are detected at lower dimension detectors for highthroughput. For highest resolution, ions are measure at the thirddimension detectors. The engineering drawings of the configuration areshown in FIG. 13A and 13B. The practical unit includes sample inlet1302, sample inlet control valve 1304, ionization source 1306 a and 1306b, and first dimension drift region 1308. The drift flow is deigned tosweep cross the second drift region 1320 1320 b and third drift region1318 a 1318 b. At drift gas inlet 1310 and 1312, a flow distributionsystem is used to assure even drift flow across the entire driftchambers. The drift gas is purge for port 1314 and 1316.

FIG. 14A and 14B shows engineering drawings of a portable system basedon the detector described in FIG. 12 and FIG. 13. The portable packageinclude, pneumatic system 1406, electronics and computer controls 1404,user interface and display 1410, battery power 1408, and a MDIMS 1402.

A modularized design approach is preferably used in the MDIMS of thepresent inventions to facilitate the provision of future upgrades. Forexample, a different ionization source may be desired for differentapplications. Such sources may be, e.g., a corona discharge,electrospray ionization or desorption electrospray ionization. Theprovision of a modular design can facilitate the changing of the ionsource.

In another aspect, the functionality of one or more of the methodsdescribed above may be implemented as computer-readable instructions ona general purpose processor or computer. The computer may be separatefrom, detachable from, or may be integrated into a MDIMS system. Thecomputer-readable instructions may be written in any one of a number ofhigh-level languages, such as, for example, FORTRAN, PASCAL, C, C++, orBASIC. Further, the computer-readable instructions may be written in ascript, macro, or functionality embedded in commercially availablesoftware, such as EXCEL or VISUAL BASIC. Additionally, thecomputer-readable instructions could be implemented in an assemblylanguage directed to a microprocessor resident on a computer. Forexample, the computer-readable instructions could be implemented inIntel 80x86 assembly language, if it were configured to run on an IBM PCor PC clone. In one embodiment, the computer-readable instructions canbe embedded on an article of manufacture including, but not limited to,a computer-readable program medium such as, for example, a floppy disk,a hard disk, an optical disk, a magnetic tape, a PROM, an EPROM, orCD-ROM (or any other type of data storage medium).

All literature and similar material cited in this application,including, but not limited to, patents, patent applications, articles,books, treatises, and web pages, regardless of the format of suchliterature and similar materials, are expressly incorporated byreference in their entirety. In the event that one or more of theincorporated literature and similar materials differs from orcontradicts this application, including but not limited to definedterms, term usage, described techniques, or the like, this applicationcontrols.

The section headings used herein are for organizational purposes onlyand are not to be construed as limiting the subject matter described inany way.

The claims should not be read as limited to the described order orelements unless stated to that effect. While the present inventions havebeen described in conjunction with various embodiments and examples, itis not intended that the present inventions be limited to suchembodiments or examples. On the contrary, the present inventionsencompass various alternatives, modifications, and equivalents, as willbe appreciated by those of skill in the art.

1. A method for detecting samples using an ion mobility spectrometricsystem, the method comprising: a. detecting ions of a compound ofinterest in a first drift chamber; b. conveying a portion of the ionsfrom the compound of interest from the first drift chamber into a seconddrift chamber; and c. confirming the ions are the compound of interestin the second drift chamber.
 2. The method of claim 1, furthercomprising a. measuring drift times of ions in a plurality of driftchambers; b. constructing a multidimensional ion mobility profile fromthe measurements; and c. identifying chemical species based on themultidimensional ion mobility profile.
 3. The method of claim 2, whereinthe multidimensional ion mobility profile is two-dimensional or threedimensional.
 4. The method of claim 2, wherein the multidimensional ionmobility profile simultaneously identifies ions in both positive andnegative polarity.
 5. The method of claim 2, wherein themultidimensional ion mobility profile simultaneously identifies ions ina single polarity.
 6. The method of claim 1, further comprises ionizingthe samples with a CFDI ionization source.
 7. The method of claim 1,further comprises confirming the ions are the compound of interest in ahigher dimension drift tube, in particular, a third, a fourth, and/or afifth.